Doping is a necessary step in semiconductor material and device processing. Doping is done by controlled introduction of certain impurity, known as dopant, into a semiconductor to modify its electrical properties such as electrical conductivity, charge carrier concentration, lifetime and type of conductivity. Doping is required for making semiconductor junctions, such as found in diodes, transistors, and others, as well as for making non-rectifying, or ohmic, electrical contact, of low contact resistance, to semiconductor material. Ohmic contact with low contact resistance is more easily fabricated on semiconductor material that has a very high charge carrier concentration.
The conventional methods of introducing dopants in silicon carbide is by ion implantation followed by activation annealing at temperatures between 1400-2200° C. in order to convert the implanted dopants into electrically active species. The higher the activation temperature, the greater the number of impurity atoms that become electrically active. Due to the high activation temperature, the surface of silicon carbide tends to decompose causing surface roughness and generating surface defects. These surface roughness and defects can have a detrimental effect on device performance. For instance, they can degrade channel mobility in silicon carbide metal-oxide-semiconductor field effect transistors to negatively impact device performance. Although activation annealing at a lower temperature is preferred, it does not provide a high enough activation percentage and is therefore inefficient. A low temperature activation annealing is also needed in the fabrication of devices based on the 3C polytype of silicon carbide grown on silicon substrate.
Another conventional method is by thermal diffusion either from gas phase or by proximity annealing. This is done by placing the impurity source in close proximity to the silicon carbide (such as flowing an impurity-containing gas over the sample, placing a solid source next to the silicon carbide sample and flowing inert gas through the chamber so that the impurity is able to be carried by the gas over the silicon carbide sample, evaporating a thin film of the metal directly on the silicon carbide sample, or evaporating a thin film of the metal directly on the silicon carbide) and annealing at temperatures in excess of 1400° C. and even up to 1900° C. The temperature has to be sufficiently high in order to get enough dopant into the sample. A lower annealing temperature, such as what is needed for 3C polytype of silicon carbide grown on silicon substrate, does not provide sufficient doping.
Another commonly used method is by in-situ doping, in which dopants are introduced during crystal growth. In thin film growth of silicon carbide by chemical vapor deposition, in-situ doping is usually done by adding impurities such as nitrogen (by using nitrogen gas or ammonia), aluminum (by using a metalorganic source such trimethylaluminum), boron (by using diborane gas), phosphorous (by using phosphine gas), etc., as part of the process gas into the growth chamber. This technique is preferred when a uniform large area coverage is required but is not suitable when only selected areas on the semiconductor wafer require doping.
A commonly used doping technique in silicon device fabrication is to use commercial spin-on dopant glass compound containing the desired dopant. Most of these spin-on dopant compounds contain both impurity dopant and silicon dioxide in an organic medium or solvent. Some spin-on dopants are also available without silicon dioxide in their formulations. In a typical procedure in silicon wafer processing, a film of spin-on dopant is deposited onto a silicon wafer and spun to obtain a thin uniform layer. The silicon wafer is baked at 100-200° C. to remove the solvent and the wafer loaded into a diffusion chamber. On heating the wafer to a temperature slightly over the melting point of the spin-on dopant, in an atmosphere containing either nitrogen gas or a gas mixture of 90% nitrogen and 10% oxygen, a smooth glassy film is formed on the surface of the wafer. Dopant from the glass is deposited on the surface of the silicon. The formation of silicate glass is to provide an inert barrier against outdiffusion of silicon. Following this step, the wafer is then cooled and removed from the diffusion chamber, and the silicate glass film is removed by wet chemical etching using dilute hydrofluoric acid solution or buffered-oxide-etch (BOE) solution. The wafer is then loaded back into the diffusion chamber and heated to about 1000-1200° C. in an atmosphere containing about 10-25% oxygen in nitrogen gas to allow diffusion, or drive-in, of the dopant into the silicon. The duration of the drive-in step depends on the desired doping depth in the material. An alternative technique of removing the silicate glass film without removing the wafer from the chamber is by heating in steam at about 1050-1250° C. for about 5-20 minutes.
Such spin-on dopants are not usually used for doping silicon carbide material because the method is not effective when employing the typical processing conditions as used in silicon device processing. For instance, nitrogen is a dopant in silicon carbide and therefore nitrogen gas is not formally used in silicon carbide annealing. However, these spin-on dopants such as those containing phosphorus are used in some silicon carbide device fabrication processes to dope other material, such as polysilicon that is used for making the gate contact on silicon carbide metal-oxide-semiconductor field effect transistor (eg. Jianwei Wan, et al., “N-Channel 3C—SiC MOSFETs on Silicon Substrate,” IEEE Electron Device Letters, Vol. 23, No. 8, August 2002, pp. 482-484.
Point defects such as vacancies and interstitials are known to play a key role in the diffusion of impurities in semiconductors. Thus, the efficiency of doping in semiconductors can be altered by the presence of these point defects. The interaction between point defects and extrinsic impurities produces localized energy states in the energy band structure of the semiconductor and alter the electronic properties of the material. Extrinsic doping by introducing an impurity produces electrically-active defect center when such impurity occupies a vacant atom site or an interstitial site between the atoms in the atomic lattice of the host semiconductor crystal. Interactions between vacancies, interstitials, and impurities to form larger defect complexes can also occur. Intentionally changing the amount of vacancies can help control the incorporation of impurities in the crystal lattice, and thus control the electrical properties of the semiconductor.
In order to manipulate the defect structure to produce a desired electrical or optical characteristics, certain impurity-related defects are preferred over others depending on the location of the defect levels within the band gap. Several methods are available to selectively favor the creation of a particular type of defects. These methods include co-implantation and co-diffusion of impurities, as well as changing the composition of the gas during epitaxial growth.
In SiC, points defects such as carbon vacancy (VC), silicon vacancies (VSi), vacancy-pairs (VSi-VC), antisite defects (CSi or SiC), or combination thereof, can interact with extrinsic impurities to modify the electrical properties of SiC. For instance, boron is known to produce two main acceptor levels in SiC.
Miyajima et al. in U.S. Pat. No. 6,133,120 review and discuss the role of boron in SiC and claim that the shallow acceptor level is due to boron occupying a silicon site (BSi) whereas the other deeper acceptor level is due boron occupying a carbon vacancy site, VC. However, several works have also shown that the deeper boron acceptor level could be due to a defect complex (BSi+VC). Due to its shallower energy level, BSi is the preferred acceptor level for more efficient dopant activation. To promote preferential creation of this BSi defect, various methods have been implemented ranging from co-implantation of boron and carbon or control of the ratio of carbon and silicon atoms in the process gas during epitaxial growth and in-situ doping by chemical vapor deposition.
A model known as “site-competition epitaxy” (D. J. Larkin, P. G. Neudeck, J. A. Powell, and L. G. Matus, Applied Physics Letters 65, 1659 (1994) explains how the incorporation of an impurity in SiC can be controlled by varying the Si/C ratio in the growth ambient during chemical vapor deposition. Varying the Si/C ratio changes the amount of silicon or carbon vacancies to selectively promote or suppress the incorporation of impurity atoms in the vacancies. In-situ doping during epitaxial growth by chemical vapor deposition is carried out at temperatures of about 1450° C. for 4H— and 6H—SiC, and about 1350° C. for 3C—SiC. However, as previously mentioned, in-situ doping by chemical vapor deposition is not suitable for selective doping of certain areas of a silicon carbide wafer.